The present invention is related to ink jet printing and, in particular, to ejectors used in generating ink droplets for such printing.
Various types of ejectors have been developed for the printheads of ink jet printers. There are "continuous stream" and "drop on demand" ejectors with nozzles having small ejection orifices for defining the size of the ejected liquid ink droplets. These suffer from many disadvantages, such as maintenance requirements due to clogged nozzles and high manufacturing costs due to the nozzles. Other kinds of ink jet printers are acoustic ink printers in which the droplets are emitted from the surface of ink reservoirs.
As the name implies, in acoustic ink printing acoustic waves are used to drive droplets from the free surface of the ink onto a recording medium. One type of acoustic ink ejector is discussed in U.S. Pat. No. 4,751,529, issued on Jan. 14, 1988 to the present inventors and another, and assigned to the present assignee. In that patent, a concave surface in the surface of a substrate is used as a lens to focus the acoustic waves at the free surface of the ink reservoir.
Another type of acoustic ink ejector is discussed in U.S. Pat. No. 4,697,195, issued to C. F. Quate et al. on Sep. 29, 1987 and also assigned to the present assignee. In that patent, surface acoustic waves are generated by interdigitated electrodes on a piezoelectric substrate. Since the electrodes are arranged concentrically in a circle, these waves form a converging cone of acoustic waves focused at the free surface of the liquid. The waves subsequently leak into an ink bath above them.
A variation of the acoustic ink ejector is found in U.S. Pat. No. 4,748,461, issued on May 31, 1988 to Scott A. Elrod, one of the inventors of the present application, and assigned to the present assignee. In that patent, focused acoustic waves are generated by transducers below the surface of a liquid-filled reservoir. The acoustic waves come to a focus at or near the surface of the liquid. The action of the transducers is modulated by a pair of circular electrodes which generate capillary waves on the liquid surface. The electrode pair can act as a switch to turn the ejection action of the acoustic waves on or off, or to control the angular trajectory of the ejected droplets.
The present invention presents a way of using capillary waves themselves to eject liquid droplets from the surface of a liquid reservoir without the use of nozzles.